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08 July 2012

Fixing segfault in Pathload

Dear readers

Couple days ago (it's 1st week of July 2012), I came across this nifty tool called Pathload. Essentially, it helps you determine the real upstream and downstream connection of yours.

Inaccidentally, when I tried to run it, it segfaulted immediately. With a bit help of gdb and trial/errors, I found and fix the bug. Here's the complete email message that I sent to its maintainer (which is I find no longer maintain it anymore) describing the problem. For those who just seek for the patch, just scroll to the end of this post (normal patch format):

Dear Constantinos

I came across this nice tool Pathload of yours today while exploring
about network management in Linux kernel. Of course, quickly I
downloaded the link to the source tarball (I use Linux -- Centos 5.x)
and compiled it.

When running it, it suddenly stopped due to segfault. After checking
the stack trace in the resulting core dump image, it leads to line 132
in client.c:
strcpy(serverList[i],buf);

My intuition suddenly told me it must out of bound char copy. Short
story short, I landed to client.h at this line:
#define MAXDATASIZE 25 // max number of bytes we can get at once

I did quick test and change the above line. Now it reads
#define MAXDATASIZE 50

I do "make clean" followed by "make". Now it runs perfectly fine as
far as I can tell.

Hopefully it means something to you for upcoming release. Meanwhile,
once again thank you for this piece of good work.

PS: strangely, without modifying any single line of source code, the
resulting binary worked fine inside GNU debugger (gdb). That's why I
suspected a race condition initially.

#define SELECTPORT 55000 // the port client will be connecting to-#define MAXDATASIZE 25 // max number of bytes we can get at once+#define MAXDATASIZE 50 // max number of bytes we can get at once #define NUM_SELECT_SERVERS 3